National Water-Quality Assessment Program

Quality of Shallow Groundwater and Drinking Water
in the Mississippi Embayment-Texas Coastal Uplands
Aquifer System and the Mississippi River Valley Alluvial
Aquifer, South-Central United States, 1994–2004

ABSTRACT

The Mississippi embayment-Texas coastal uplands
aquifer system is an important source of drinking water, providing
about 724 million gallons per day to about 8.9 million
people in Texas, Louisiana, Mississippi, Arkansas, Missouri,
Tennessee, Kentucky, Illinois, and Alabama. The Mississippi
River Valley alluvial aquifer ranks third in the Nation for
total withdrawals of which more than 98 percent is used for
irrigation. From 1994 through 2004, water-quality samples
were collected from 169 domestic, monitoring, irrigation,
and public-supply wells in the Mississippi embayment-Texas
coastal uplands aquifer system and the Mississippi River
Valley alluvial aquifer in various land-use settings and of
varying well capacities as part of the U.S. Geological Survey's
National Water-Quality Assessment Program. Groundwater
samples were analyzed for physical properties and about
200 water-quality constituents, including total dissolved
solids, major inorganic ions, trace elements, radon, nutrients,
dissolved organic carbon, pesticides, pesticide degradates, and
volatile organic compounds.

The occurrence of nutrients and pesticides differed
among four groups of the 114 shallow wells (less than or equal
to 200 feet deep) in the study area. Tritium concentrations in
samples from the Holocene alluvium, Pleistocene valley trains,
and shallow Tertiary wells indicated a smaller component of
recent groundwater than samples from the Pleistocene terrace
deposits. Although the amount of agricultural land overlying
the Mississippi River Valley alluvial aquifer was considerably
greater than areas overlying parts of the shallow Tertiary and
Pleistocene terrace deposits wells, nitrate was rarely detected
and the number of pesticides detected was lower than other
shallow wells. Nearly all samples from the Holocene alluvium
and Pleistocene valley trains were anoxic, and the reducing
conditions in these aquifers likely result in denitrification of
nitrate. In contrast, most samples from the Pleistocene terrace
deposits in Memphis, Tennessee, were oxic, and the maximum
nitrate concentration measured was 6.2 milligrams per liter.
Additionally, soils overlying the Holocene alluvium and
Pleistocene valley trains, generally in areas near the wells, had
lower infiltration rates and higher percentages of clay than
soils overlying the shallow Tertiary and Pleistocene terrace
deposits wells. Differences in these soil properties were
associated with differences in the occurrence of pesticides.
Pesticides were most commonly detected in samples from
wells in the Pleistocene terrace deposits, which generally had
the highest infiltration rates and lowest clay content.

Median dissolved phosphorus concentrations were 0.07,
0.11, and 0.65 milligram per liter in samples from the shallow
Tertiary, Pleistocene valley trains, and Holocene alluvium,
respectively. The widespread occurrence of dissolved
phosphorus at concentrations greater than 0.02 milligram per
liter suggests either a natural source in the soils or aquifer
sediments, or nonpoint sources such as fertilizer and animal
waste or a combination of natural and human sources.
Although phosphorus concentrations in samples from the
Holocene alluvium were weakly correlated to concentrations
of several inorganic constituents, elevated concentrations
of phosphorus could not be attributed to a specific source.
Phosphorus concentrations generally were highest where
samples indicated anoxic and reducing conditions in the
aquifers. Elevated dissolved phosphorus concentrations in
base-flow samples from two streams in the study area suggest
that transport of phosphorus with groundwater is a potential
source contributing to high yields of phosphorus in the lower
Mississippi River basin.

Water from 55 deep wells (greater than 200 feet deep)
completed in regional aquifers of Tertiary age represent a
sample of the principal aquifers used for drinking-water
supply in the study area. The wells were screened in both
confined and unconfined parts of these aquifers, and tritium
samples collected from 11 wells did not indicate a significant
component of recent water. The quality of the water generally
was good, and concentrations of most inorganic constituents
were less than water-quality criteria for drinking water.
Relatively few pesticides or volatile organic compounds were
detected. Secondary drinking-water standards were exceeded
for iron, manganese, and total dissolved solids in a total of 20,
17, and 10 wells, respectively. Fluoride was detected in one
sample at a concentration greater than its secondary maximum
contaminant level. Radon activities were greater than
300 picocuries per liter in samples from 10 wells screened
in the middle Wilcox aquifer in northwestern Louisiana.
While not a drinking-water concern, elevated phosphorus
concentrations also were measured in the drinking-water
supply aquifers. A relation between phosphorus concentrations
and dissolved solids and fluoride suggests a geologic source of
phosphorus in these deep wells.

Groundwater withdrawals from these regional aquifers
have lowered water levels, which increases the potential
of movement of shallow groundwater to deep aquifers.
Water-level data for a few wells in the Pleistocene terrace
deposits in the Memphis area suggest a hydraulic connection
between this shallow aquifer and the deeper middle Claiborne
aquifer, locally named the Memphis aquifer, that is used for
public supply. Additionally, pesticides and volatile organic
compounds were detected in samples from wells screened in
shallow and deeper portions of the middle Claiborne aquifer,
further illustrating the potential vulnerability of this source of
drinking water.

Contents

The Quality of Drinking Water in the Mississippi Embayment-Texas Coastal Uplands
Aquifer System

Summary

References

Appendix 1. Detection frequencies of volatile organic compounds, 1994–2004, in the
Mississippi embayment-Texas coastal uplands aquifer system and
Mississippi River Valley alluvial aquifer with and without the 0.2 microgram
per liter assessment level